Testing the climate-envelope hypothesis in conifers: Sapling response to early snowmelt along an elevational gradient

Projections of responses to climate change often invoke the climate-envelope hypothesis, which assumes that all individuals in a species' range are adapted to the same climatic optimum. By neglecting the potential for genetic adaptation to climate at the population level, these projections risk underestimating the effects of climate change on individuals at different locations along elevational or latitudinal gradients. To test this assumption, we assessed the response of two conifer species to a 5-year shift in snowmelt date along an elevational gradient in western Colorado. By measuring internode distance, we were able to record the age and growth of saplings across a 12-year period at 22 locations spanning 815m in elevation. By regressing change in growth (between periods of late and early snowmelt) against mean snowmelt date at each location, our results suggest that assuming a single climatic optimum is valid in this system. While the climate-envelope hypothesis holds in these species with high gene flow along an elevational gradient, applying a similar approach across a fragmented or latitudinal gradient is recommended for future work.